The present invention relates to a sliding member used as bearings for automobiles, ships, aircraft or general industrial machines.
As such type of sliding member, there has been known one comprising a steel back, a bearing alloy layer being made of a copper or aluminum alloy for example and provided on the steel back, and a lead alloy plating film formed on the surface of the bearing alloy layer via an intermediate plating layer.
In recent engines of motor vehicle, under demands for cost reduction, shafts made of cast iron such as nodular graphite cast iron have come into popular use. Such cast iron shafts, however, have a problem that the surface layer of bearings, which supports the shaft, tends to suffer early wear due to burrs existing around graphite grains on the shaft surface, resulting in that the intermediate plating layer or the bearing alloy layer exposes to cause rapid seizure.
In the case of commonly used steel shafts, they tend to be used under a high speed and a high load, so that there has arisen the same problem, as mentioned above, of early wear of the surface layer of bearings.
In order to solve such a problem, the present inventors proposed previously, in a previous application of JP-A-2-49132, to use a composite plating film as an overlay of the sliding member. The former invention relates to forming a composite plating film on a bearing alloy layer, the composite plating film being made of a lead alloy containing 2 to 30 mass % in total of at least one element selected from Sn, In, Sb and Cu and 0.3 to 25 vol % of co-deposited inorganic particles, wherein the lead matrix is protected by the relatively hard inorganic substance in the composite plating film thereby preventing the surface layer of the sliding member from wear.
The composite plating film, however, has a problem that it has a rough surface due to the presence of inorganic particles, thereby causing an increase of wear of the mating shaft although the wear resistance property of the sliding member is improved.
Thus, the present inventors studied the problem and reached a conception that it is possible to improve conformability between the sliding member and the mating shaft in an initial stage at running-in by reducing an amount of a dispersion of inorganic particles in the surface layer of the lead alloy composite plating film and increasing an amount of a dispersion of inorganic particles gradually or continuously in proportion to the depth of the composite plating film, whereby assuring subsequent wear resistance of the sliding member. A patent application of JP-A-3-97090 is based on this idea.
According to JP-A-3-97090, the lead alloy composite plating film contains copper in order to cope with a rise of bearing surface pressure for the sake of developing high power engines. However, when copper is added to the lead alloy composite plating film, there arise a potential problem of occurrence of sudden seizure because of inferior conformability of the composite plating film due to hard copper, although the composite plating film can be improved in mechanical strength.
Accordingly, an object of the present invention is to provide a sliding member having a composite plating film made of a lead alloy containing copper and inorganic particles dispersed in the lead alloy, which does not suffer seizure at an early stage, is capable of minimizing wear of its own and the mating member, and has excellent fatigue resistance property, even if it is used under a high speed and a high load in operation of an engine.
Under the above object, according to the present invention, there is provided a sliding member comprising a bearing alloy layer and a composite plating film provided on the bearing alloy layer, wherein the composite plating film is made of a lead alloy containing 0.1 to 10 mass percent in total of copper and 0.3 to 25 volume percent in total of co-deposited inorganic particles, and the outermost surface layer of the composite plating film, which has a thickness proportion of 10 to 40% to the entire thickness of the composite plating film, does not contain inorganic particles and copper.
Since the outermost surface layer of the lead alloy composite plating film does not contain inorganic particles, it is possible to lower the coefficient of friction at the running-in stage or the initial wear stage, whereby it is possible to reduce wear of not only the mating member but also that of the sliding member itself. Also, as the outermost surface layer of the composite plating film contains no copper or, if any, only a trace amount of copper, it is possible to improve the composite plating film in initial conformability at the running-in stage, whereby the problem of sudden seizure of the plating film can be solved.
In usual, the lead alloy composite plating film has a thickness of 10 to 30 xcexcm in the case where the sliding member is a component of motor vehicle engines. The thickness, however, is not restrictive, in some cases, for example, in the case of the sliding member for ship engines, it may be 50 to 100 xcexcm.
When the sliding member and the mating member are in the stage of normal wear after having been well conformed to each other in the initial stage of operation, since the sub-layer containing inorganic particles and copper beneath the outermost surface layer is exposed, the sliding member is prevented from wear and also hard to fatigue.
If the thickness of the outermost surface layer containing none of inorganic particles and copper is less than 10% of the composite plating film thickness, the initial conformability of the composite plating film is inferior. And, if the thickness of the outermost surface layer containing none of inorganic particles and copper exceeds 40% of the composite plating film thickness, it is impossible to expect the effect of improving wear and fatigue resistance of the composite plating film.
The lead alloy of the composite plating film preferably contains 2 to 30 mass % in total of at least one of Sn and In. Sn and In improve the outermost surface layer of the composite plating film in mechanical strength and corrosion resistance to lubricating oils. If the total content of the elements is less than 2 mass %, the composite plating film is low in mechanical strength and corrosion resistance for a long term. If the content exceeds 30 mass %, the composite plating film is extremely deteriorated in mechanical strength at high temperature. Thus, the content of the above elements should be 2 to 30 mass %, preferably 3 to 25 mass %.
The sliding member of the present invention is of a multilayer structure comprising a steel back, a copper-based or aluminum-based bearing alloy layer, and an overlay layer of composite plating film made of a lead alloy. Preferably, an intermediate plating layer is interposed between the composite plating film and the copper-based or aluminum-based bearing alloy layer.
The intermediate plating layer consists of any one metal selected from the group of Ni, Ag, Cu, Co and an alloy containing the metal as a major component. Ni and Co serves for preventing Sn and/or In in the composite plating film as a surface layer from diffusing into the base alloy (i.e. bearing alloy) thereby preventing deterioration of strength and corrosion resistance of the matrix of the base alloy. With regard to Cu and Ag, advantageously they produce a diffusion alloy together with In contained in the composite plating film in view of sliding characteristics, and further advantageously it is possible to improve the anti-seizure property by plating Cu and/or Ag on the aluminum alloy.